In principle, there are, for such “in-line” sensors serving for measuring a fluid flowing in a pipeline, only two kinds of measuring tubes, these being, on the one hand, straight measuring tubes, and, on the other hand, arbitrarily curved, or even coiled, measuring tube loops, among which the U-shaped ones are the preferred tube forms. Thus, U.S. Pat. Nos. 4,127,028, 4,524,610, 4,768,384, 4,793,191, 4,823,614, 5,253,533, 5,610,342, 6,006,609 and the not pre-published European Patent Application 01 112 546.5 of the present assignee describe vibration-type sensors, especially for producing mass-flow-dependent, Coriolis forces, and/or for producing viscosity-dependent frictional forces, in flowing fluids. Such sensors have at least one measuring tube for guiding a fluid, the measuring tube having an inlet end and an outlet end, and vibrating at least at times, the measuring tube. The measuring tube communicating, by way of a first tube segment leading into the inlet end and a second tube segment leading into the outlet end, with a pipeline connected for allowing the fluid to flow through the measuring tube, and during operation executing mechanical oscillations about an oscillation axis imaginarily connecting the two tube segments. Included is a support for the oscillation-permitting holding of the measuring tube, which support is secured to the first tube segment by means of a first transition piece and to the second tube segment by means of a second transition piece.
Especially in the case of Coriolis mass flow sensors serving for the measuring of mass flow rates, mostly, due to reasons of symmetry, two measuring tubes are employed when using either of the two types of measuring tubes, the straight ones or the looped ones. The two tubes extend, when at rest, parallel to one another, in two parallel planes and, most often, the fluid flows through them in parallel, as well. For the one of the two variants, that with two parallel, straight tubes, reference can be made, purely by way of example, to the U.S. Pat. Nos. 4,768,384, 4,793,191 and 5,610,342, while, for the other, that with two parallel, especially identically-shaped, U-shaped tube loops, see e.g. U.S. Pat. No. 4,127,028.
Besides the aforementioned types of double-tube mass flow sensors working on the Coriolis principle, a further type of sensor has established itself in the market for quite some time now, namely that which uses only a single, straight, or bent, measuring tube. Such sensors are described e.g. in the U.S. Pat. Nos. 4,524,610, 4,823,614, 5,253,533, 6,006,609 and in the not pre-published, European patent application 01 112 546.5.
Additionally, U.S. Pat. No. 4,823,614 describes that each end of the one measuring tube is inserted in a matching bore of an inlet, respectively outlet, transition piece and fixed therein by welding, soldering or brazing; see the material beads visible in some of the figures. The transition pieces are, in turn, secured in an external support.
As already discussed in U.S. Pat. No. 5,610,342, the needed heat supply to the securement locations of the measuring tube to the transition pieces during the mentioned welding, soldering or brazing can produce, upon cooling, residual mechanical stresses, which can lead to stress corrosion cracking, especially when fluids are being measured, which attack the material of the measuring tube to a greater or lesser degree. For eliminating this danger of stress corrosion cracking as completely as possible for measuring tubes of Coriolis mass flow sensors, an improved method of securing measuring tubes in transition pieces has been likewise proposed in U.S. Pat. No. 5,610,342, wherein each end of the measuring tube is inserted in a corresponding bore of an inlet, respectively outlet, transition piece and pressed without the introduction of heat against the wall of the bore by means of a rolling tool placed in such end. A rolling tool appropriate for this method is described, for example, in U.S. Pat. No. 4,090,382 concerning the securing of tubes of boilers or heat exchangers.
Investigations of sensors manufactured by this method have shown, however, that the usually different expansion behaviors of the aforementioned transition pieces and the measuring tube clamped in each can lead to the clamping forces exerted by the transition pieces on the measuring tube falling below a critical value in the presence of temperature fluctuations, especially in the case of possible temperature shocks, such as can occur e.g. during regularly executed cleaning operations using extremely hot washing liquids. This, in turn, can mean that transition piece and measuring tube lose can occur by the mechanical contact brought about by the rolling, due to thermally-caused expansions, so that the support can then twist about the aforementioned oscillation axis relative to the measuring tube. For the then no longer certainly excludable case of such a twisting of the support, especially in the case of sensors with measuring tubes which during operation also execute torsional oscillations about the oscillation axis, a replacement of the entire measuring device becomes practically unavoidable.